Mere exposure to task-irrelevant coherent motion leads to performance improvement on the motion (Watanabe et al, 2001). The underlying mechanism for such task-irrelevant perceptual learning (TIPL) has yet to be clarified. TIPL could arise as a result of distraction from a central task or attentional leakage from location of a task target. We tested whether any of these possibilities is true. In Experiment 1, hierarchical letters (Navon, 1977) were presented at the center of a display. Participants (n=4) were asked to recognize either large compound or small component letters in a block design. A task-irrelevant 5% coherent motion display was presented in a periphery. Given a fixed high contrast, small (harder) letters induced stronger TIPL than large (easier) letters. In Experiment 2 (n=8), only small letters were used as targets for recognition, with two letter contrasts alternating across blocks. Given a fixed scale of task targets, low-contrast (harder) letters induced stronger TIPL than high-contrast (easier) letters. In Experiment 3 (n=9), the task was to recognize regular letters at the center, in large or small size, again using a block design. Given a fixed task difficulty controlled by the staircase method throughout training, small letters did not induce weaker TIPL than large letters. In all the experiments training lasted five days. Given that with a harder task, the degree of involvement of focused attention is greater and the attentional window size is smaller toward the task targets (e.g., Ikeda & Takeuchi, 1975; Rees et al., 1997; Yi et al., 2004), our results cannot be explained by the mere involvement of the traditional focused attention concept in task-irrelevant processing. The results are rather in accordance with the model in which a harder task at a central field more greatly boosts signals outside a window of focused attention and leads to greater TIPL.